The present invention relates to a valve arrangement for hydraulically actuated machinery installations in mining, especially for powered support assemblies in underground mining, with one or more hydraulically actuated working valves for controlling one or more hydraulically actuated machine units of the machinery installation and with at least one electro-hydraulic pilot valve assigned to the working valve or valves, controlling it or them.
Valve arrangements of this construction have many different areas of application in underground mining, for instance for the control of hydraulic cylinders in powered support assemblies, which in the continuously advancing mining of the face have to be regularly withdrawn, traversed and reset. The supply and withdrawal of the hydraulic fluid to the extending and withdrawing hydraulic cylinders is controlled by the working valves, which in turn are have to be switched by the pilot valves, since the power required to switch the working valves cannot be applied by the electro-magnets driven within the intrinsically safe power region, which are only permitted for application in underground mining.
The switching of the working valves is effected thereby by means of the high pressure hydraulic fluid itself. In the known valve circuits the arrangement is chosen such that the electro-hydraulic pilot valves are connected to the same hydraulic fluid supply as the large users, the normally applied threexe2x80x94twoxe2x80x94way valves, and are thus subject to the same pumping pressure made available by the hydraulic fluid pumps. This known arrangement has operated reliably in most machine installations for the control of their hydraulic units. However in large users, for instance in very large powered support assemblies, which are designed for a high loading and are provided with correspondingly large props and cylinders, it can arise that a large amount of hydraulic fluid is required within a short time, for instance when several users have to be controlled simultaneously, which has the consequence that the pressure in the high pressure system delivering hydraulic fluid falls significantly. This has in turn the consequence that the pilot valves begin to flutter due to varying or too little hydraulic pressure, this means that their desired position is not continuously maintained, but they travel open and shut, whereby the working valves also are not maintained in the required switching position and thereby an even effect is not provided on the loads controlled by them.
It is an aim of the present invention to overcome the aforementioned disadvantage and to produce a valve arrangement of the construction described above, with which uneven operation of the valves, that is their fluttering, is reliably overcome, so that an even actuation of the hydraulically operable machine units, especially a smooth in and out movement of the prop and the cylinders is provided.
Accordingly the present invention is directed to a valve arrangement as described in the opening paragraph of the present specification in which a dedicated pressure medium supply, separated or separable from the hydraulic fluid supply of the working valve or valves is attached to the pilot valve or valves.
In accordance with the invention, the pilot valves thus have their own pressure medium supply, which can be connected to the hydraulic fluid supply for the working valves, but which can however in need be separable from this or even fully independent of it. Pressure changes in the hydraulic fluid supply for the working valves do not therefore have an effect on the pressure medium supply for the pilot valves, but a largely constant hydraulic fluid pressure acts on them, so that it is ensured that the pilot valves do not, as a result of falling off of pressure in the hydraulic fluid supply to the working valves, come into the unstable operating condition described above, but retain the desired valve setting over the whole desired time period. It is thereby ensured that even under strong pressure variations in the hydraulic fluid system, the working valves are retained in their desired open or closed setting, so that the hydraulic fluid is continuously fed to the appropriate machine units.
Advantageously a pressure storage device is attached to the pressure medium supply for the pilot valve or valves. The pressure storage device can comprise a simple hydraulic tube, in which a part of the pressure medium of the pressure medium supply is stored and, due to the at least minimal available compressibility of the hydraulic tube, tube, forms a sort of pressure buffer, which ensures an even pressure in the whole pressure medium supply system. Especially the pressure storage device comprises a pressure equalising container.
In a preferred embodiment the pressure storage device has a gas-filled pressure bubble. Due to the essentially higher compressibility of the gas ensures an especially good pressure equalisation and itself ensures an almost unchanging pressure in the pressure medium supply circuit, when all the pilot valves are operated simultaneously and the volume of the pressure medium system thereby measurably changes.
The gas filling of the pressure bubble comprises advantageously an inert gas, advantageously nitrogen, so that it is ensured that no inflammable gas mixture can form within the pressure storage device.
Preferably the pressure medium supply for the pilot valve is, or can be, connected to the hydraulic fluid supply for the working valve or valves with a stop valve connected between them. In this development the required hydraulic pressure for the pilot valve or valves is built up in a similar manner as in previously proposed valve arrangements, as long as the high pressure required for this is available in the hydraulic fluid supply for the working valves. The cut off valve ensures that a falling off of pressure in the hydraulic fluid supply for the working valves has no influence on the pressure medium supply for the pilot valves, since with the cut off valve shut, the two fluid systems are without any connection to each other. In a preferred embodiment the stop valve is configured as a non-return valve. Thus automatically separating the pressure medium system for the pilot valves from the remaining hydraulic fluid supply, when a falling off of pressure occurs in the latter. On the other hand, it is ensured by this arrangement, that the pressure in the pressure medium supply for the pilot valves is always at least as high as the pressure in the rest of the hydraulic system.
An example of a valve arrangement made in accordance with the present invention will be described below with regard to the attached drawing the only FIGURE of which shows a powered face support assembly in a schematic hydraulic circuit plan.
In the FIGURE the overall valve arrangement 10 serves to control the hydraulic cylinders present in an advancing powered support assembly, with whose help the support assembly can be, in operation, withdrawn, traversed and reset again, whereby the cylinders can be moved in and out over a large distance, so as to provide adjustment of the powered support assembly to the changing mining conditions as the face progresses.